Synthesis of Zn(NH3) (CO3) inorganic helical framework and its use for selective separation of carbon dioxide

ABSTRACT

A novel one-pot solvothermal reaction based on urea hydrolysis to synthesize single crystals of the Zn(NH3)(CO3) inorganic helical framework and its applications in selective CO2 separation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of the filing ofU.S. Provisional Patent Application Ser. No. 62/219,565, entitled“SYNTHESIS OF ZN(NH₃)(CO₃) INORGANIC HELICAL FRAMEWORK AND ITS USE FORSELECTIVE SEPARATION OF CARBON DIOXIDE”, filed on Sep. 16, 2015, and thespecification and claims thereof are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No.N00014-10-1-0711 awarded by the Office of Naval Research (ONR). Thegovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION Field of the Invention (Technical Field)

The present invention relates generally to inorganic helical frameworks,their synthesis and use, and more particularly, to synthesis and use ofamminezinc carbonate (“Zn(NH₃)(CO₃)”) inorganic helical frameworks.

The environmental issues associated with CO₂ emissions have made carboncapture and storage (CCS) a major research area for materials scientistsand engineers, and among the most promising materials for CCS areadsorbents, which are characterized by the two main characteristics ofadsorption capacity and separation selectivity. However, despite thesynthesis of materials with such high adsorption capacities, theseparation of CO₂ from other gases remains an ongoing challenge. CO₂, inthe gaseous streams of the natural gas, hydrogen production, and powergeneration industries, typically is mixed with other gases such as N₂,CH₄, H₂, and O₂. Therefore, effective adsorbents must have not only highadsorption capacities, but also high selectivity for CO₂, ideally at theoperating conditions of the relevant CO₂ sources. For instance, the fluegas of power plants is released at ambient temperature and pressure, andis comprised mainly of N₂ and CO₂ at a ratio of 85:15.

Ammoniacal ammonium carbonate solution has been widely used in theleaching of metals like nickel, copper, cobalt, iron, silver, manganese,and zinc from ores. The leaching system is usually made by a reactionbetween metal ore and ammonium carbonate in ammonia solution.Alternatively, ammonium carbamate and urea in water can be used.Ammonium and carbonate ions are released at temperatures above 363 K asa result of hydrolysis to form the leachant. The leaching process isbased on the selective dissolution of the target metal ion from apolymetallic mixture. Such an understanding allows the adoption of a setof extraction reaction parameters whose establishment results in theformation of a soluble target metal leachate from the ore without animpact to other metals, which are left in the residue.

When ammoniacal ammonium carbonate (Zn—NH₃—CO₃ ²⁻ system) or ammoniasolution (Zn—NH₃—H₂O system) is the leachant, zinc amine complex is thesoluble leachate that is produced. In the Zn—NH₃—H₂O system, Zn(NH₃)₄ ²⁺is stable in the pH range of 8 to 11 when the total concentration of NH₃is 1M. Outside of this pH range, the stable species is zinc hydroxide.Additionally, when the concentration of ammonia increases, the stabilityregions of the solid phase shrink and eventually disappear. The pH rangeat which zinc tetraammine complex is stable in the Zn—NH₃—CO₃ ²⁻ systemis approximately the same for the Zn—NH₃—H₂O system. The extractionreaction for zinc oxide in an ammonia-ammonium carbonate system isrepresented by Equation 1:ZnO+(NH₄)₂CO₃+2NH₄OH→Zn(NH₃)₄ ²⁺+CO₃ ²⁻+3H₂O  (1)

Although the product of Equation 1 gives a Zn:NH₃ ratio of 1:4, t thisratio may vary from 1:1 to 1:6, depending on the concentration ofammonia in the solution. Therefore, several compounds ofZn(NH₃)_(X)(CO₃) are possible. The compound where X=1, Zn(NH₃)(CO₃), isnot soluble in water and precipitates. In the Zn—NH₃—CO₃ ²⁻ system, thiscompound is a solid phase, as is the Zn₅(OH)₆(CO₃)₂ that forms at higherzinc concentrations.

Synthesis of Zn(NH₃)(CO₃) was first reported via a two-step procedure:production of Zn(NH₃)₄(CO₃) by reaction between zinc oxide or zinccarbonate and ammonium carbonate in ammonia solution and, then,precipitation of Zn(NH₃)(CO₃) by heating, air blowing, or saturating ofZn(NH₃)₄(CO₃) solution with CO₂ at high pressure in order to lower theammonia content of the solution. Later, single crystal x ray diffractionanalysis (SC-XRD) revealed the unit cell characters of the crystallinestructure. Based on the analysis, the lattice had orthorhombic systemwith unit cell parameters of a=9.130 Å, b=5.498 Å, and c=7.593 Å andspace group of Pna2₁ (ICSD #41113).

Embodiments of the present invention provide methods to synthesizeZn(NH₃)(CO₃) and its application in CCS.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention comprise a method of synthesizingZn(NH3)(CO3) in a one-pot solvothermal reaction based on urea hydrolysiscomprising: constructing a Zn—NH3-CO32− system comprising a zinc sourceand urea solutions in the mixture of N,N-Dimethylformamide (DMF) andwater; heating at between approximately 80° C. and approximately 100°C.; solvothermal aging at between approximately 120° C. andapproximately 160° C.; and growing single crystals of Zn(NH3)(CO3). Inone embodiment the zinc source is selected from the group consisting ofzinc acetate, zinc nitrate, zinc chloride, and zincsulfate. In oneembodiment the heating is carried out for between approximately 1 hourand approximately 10 hours.

In one embodiment, the constructing a Zn—NH3-CO32− system a comprisesdissolving the urea in the DMF and water; heating up at betweenapproximately 85° C. and approximately 95° C. for between approximately1 hr. and approximately 6 hrs.; adding zinc acetate solution is tohydrolyzed urea solution; and heating the resulting mixture up forbetween approximately 15 minutes to approximately 90 minutes untilprecipitation is completed. In one embodiment, the mixture of zincacetate and urea is heated until precipitation is completed. In oneembodiment, the solvothermal aging is carried out for betweenapproximately 1 and approximately 7 days. In one embodiment, the growingsingle crystals step further comprises washing the crystals in with DMFand soaking the crystals in chloroform. Preferably, the Zn—NH3-CO32−system is maintained at the pH of between approximately 8 andapproximately 11. Preferably, the Zn—NH3-CO32− system comprisesconcentrations of zinc and carbonate ions at about 0.1M. Preferably, theurea hydrolysis slowly pumps NH3 and Carbonate to the reaction medium.

In one embodiment DMF is substituted for another solvent with a highboiling point, for example, N-Methyl-2-pyrrolidone (NMP), dimethylsulfoxide (DMSO), hexamethylphosphoramide (HMPA), pyridine (Py), andgamma-Butyrolactone (GBL).

Embodiments of the invention further comprise a method to capturing CO2comprising: adsorption screening of CO2 from a gas mixture byZn(NH3)(CO3). In one embodiment, the gas mixture is a CO2/O2 mixture. Inone embodiment, the capture is for an oxy-fuel CO2 process by pressureswing adsorption and preferably the Pads/Pdes is about 5 atm/1 atm.

In a different embodiment, the gas mixture is CO2/CH4. In oneembodiment, the capture is for landfill gas process under pressure swingadsorption, and the Pads/Pdes is preferably about 5 atm/1 atm.

In another embodiment, the capture is for landfill gas process undervacuum swing adsorption and preferably the Pads/Pdes is about 1 atm/0.1atm.

Embodiments of the invention comprise a method of separating O2 from N2in air comprising: adsorption screening of O2 in air by Zn(NH3)(CO3). Inone embodiment, the Pads/Pdes is about 5 atm/1 atm.

Further scope of applicability of the present invention will be setforth in part in the detailed description to follow, taken inconjunction with the accompanying drawings, and in part will becomeapparent to those skilled in the art upon examination of the following,or may be learned by practice of the invention. The objects andadvantages of the invention may be realized and attained by means of theinstrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, illustrate one or more embodiments of the presentinvention and, together with the description, serve to explain theprinciples of the invention. The drawings are only for the purpose ofillustrating one or more embodiments of the invention and are not to beconstrued as limiting the invention. In the drawings:

FIG. 1 is a photograph of Zn(NH₃)(CO₃) single crystals;

FIG. 2 shows powder x ray diffraction (“PXRD”) pattern of as-synthesizedproduct as well as simulated pattern of Zn(NH₃)(CO₃);

FIG. 3 shows adsorption isotherm for N₂ uptake at 293, 273, 263, and 253K;

FIG. 4 shows adsorption isotherm for H₂ uptake at 293, 273, 263, and 253K;

FIG. 5 shows adsorption isotherm for CO₂ uptake at 313, 293, 273, 263,and 253 K;

FIG. 6 shows adsorption isotherm for CH₄ uptake at 293, 273, 263, and253 K;

FIG. 7A shows adsorption isotherm for O₂ uptake at 293, 273, 263, and253 K;

FIG. 7B shows the Performance of Zn(NH₃)(CO₃) and other selectedadsorbents in PSA-based oxy-fuel CO₂ purification. The highest value ofeach parameter is bolded;

FIG. 8 shows the adsorption branches of the isotherms of CO₂, O₂, H₂,CH₄, and N₂ isotherms at 293 K for comparison;

FIG. 9 shows the variation of composition of adsorbed phase andseparation selectivity by Zn(NH₃)(CO₃), with pressure calculated by IASTmethod for binary mixture of CO₂/CH₄ (10/90) at 273 and 293 K;

FIG. 10 shows the variation of composition of adsorbed phase andseparation selectivity by Zn(NH₃)(CO₃), with pressure calculated by IASTmethod for binary mixture of CO₂/CH₄ (50/50) at 273 and 293 K;

FIG. 11 show the variation of composition of adsorbed phase andseparation selectivity by Zn(NH₃)(CO₃), with pressure calculated by IASTmethod for binary mixture of CO₂/H₂ (40/60) at 273 K and 293 K;

FIG. 12 shows the variation of composition of adsorbed phase andseparation selectivity by Zn(NH₃)(CO₃), with pressure calculated by IASTmethod for binary mixture of CO₂/N₂ (10/90) at 273 K and 293 K;

FIG. 13 shows the variation of composition of adsorbed phase andseparation selectivity by Zn(NH₃)(CO₃), with pressure calculated by IASTmethod for binary mixture of CO₂/O₂ (90/10) at 273 K and 293 K;

FIG. 14 shows the variation of composition of adsorbed phase andseparation selectivity by Zn(NH₃)(CO₃), with pressure calculated by IASTmethod for binary mixture of H₂/N₂ (50/50) at 273 K and 293 K;

FIG. 15 shows the Polynomial fittings of CO₂ (P,q) isotherm data;

FIG. 16 shows sample isosteres of CO₂ and temperature windows ofdiffusion- and adsorption-controlled domains;

FIG. 17 shows sample isosteres of N₂ and temperature windows ofdiffusion- and adsorption-controlled domains;

FIG. 18 shows sample isosteres of H₂ and temperature windows ofdiffusion- and adsorption-controlled domains;

FIG. 19 shows sample isosteres of O₂ and temperature windows ofdiffusion- and adsorption-controlled domains;

FIG. 20 shows sample isosteres of CH₄ and temperature windows ofdiffusion- and adsorption-controlled domains;

FIG. 21 shows Isosteric heat of adsorption of CO₂ calculated fordiffusion-controlled (diamond) and adsorption-controlled domains(square);

FIG. 22 shows Isosteric heat of adsorption of N₂ calculated fordiffusion-controlled (diamond) and adsorption-controlled domains(square);

FIG. 23 shows Isosteric heat of adsorption of H₂ calculated fordiffusion-controlled (diamond) and adsorption-controlled domains(square);

FIG. 24 shows Isosteric heat of adsorption of O₂ calculated fordiffusion-controlled (diamond) and adsorption-controlled domains(square);

FIG. 25 shows Isosteric heat of adsorption of CH₄ calculated fordiffusion-controlled (diamond) and adsorption-controlled domains(square);

FIG. 26 shows Isosteric heats of adsorption of CO₂, N₂, H₂, O₂, and CH₄at diffusion-controlled domain;

FIG. 27 shows Isosteric heats of adsorption of CO₂, N₂, H₂, O₂, and CH₄at adsorption-controlled domain;

FIG. 28 shows N₂ adsorption isotherm at 77 K by Zn(NH₃)(CO₃), used tocalculate BET surface area. Filled and open symbols represent adsorptionand desorption data, respectively;

FIG. 29A shows the crystalline structure of Zn(NH₃)(CO₃). CO₃ trigonalplanars and ZnO₃N tetrahedra are shown in gray and green, respectively.The circular arrows show the spiral directions of the folded helices.The (ZnOCO)₂ and (ZnOCO)₄ helices are visualized in orange and blue,respectively. The border of the helices is shown in yellow;

FIG. 29B shows the folded helices are shown in detail. Ammine ligandsare eliminated for better visualization in this schematic; and

FIG. 29C shows the projection of crystalline structure in the bdirection. The two types of microchannels with 8 and 16 components inthe Zn(NH₃)(CO₃) helical framework are shown in yellow and cyan,respectively. (Legend: (Green: Zn; Gray: C; Red: O; Cyan: N; Magenta:H)).

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the embodiments ofthe invention. However, it will be understood by one of ordinary skillin the art that the embodiments may be practiced without these specificdetails. For instance, well known operation or techniques may not beshown in detail. Technical and scientific terms used in this descriptionhave the same meaning as commonly understood to one or ordinary skill inthe art to which this subject matter belongs.

In one embodiment, Zn(NH₃)(CO₃) is synthesized through a single-potapproach based on urea hydrolysis and solvothermal aging. In thisembodiment, zinc acetate and urea solutions in the mixture ofN,N-Dimethylformamide (DMF) and water are preferably used to construct aZn—NH₃—CO₃ ²⁻ system that upon heating at between approximately 80° C.and approximately 100° C., more preferably between approximately 85° C.and approximately 95° C., and most preferably between approximately 88°C. and approximately 92° C., and afterwards solvothermal aging atbetween approximately 120° C. and approximately 160° C., more preferablybetween approximately 130° C. and approximately 150° C., and mostpreferably between approximately 135° C. and approximately 142° C.,leads to the growth of large single crystals of Zn(NH₃)(CO₃). In oneembodiment DMF is substituted for another solvent with a high boilingpoint, for example, N-Methyl-2-pyrrolidone (NMP), dimethyl sulfoxide(DMSO), hexamethylphosphoramide (HMPA), pyridine (Py), andgamma-Butyrolactone (GBL).

Thermodynamic calculations show that zinc ammine is the dominant stablespecie in Zn—NH₃—CO₃ ²⁻ system at pH of 10 when concentration of totalzinc and carbonate ions are below 0.1M respectively. As theconcentration of the zinc or carbonate ions increase, the zinc amminefraction exponentially decreases and Zn₅(OH)₆(CO₃)₂ precipitates.Therefore, as long as synthesis of Zn(NH₃)(CO₃) is taking place, theconcentration of zinc and carbonate sources in the reaction mixtureshould preferably not exceed about 0.1M.

In addition to the zinc and carbonate sources concentration, theconcentration of ammonia in the reaction mixture is preferably lowenough to control the pH between 8 and 11 to crystalize Zn(NH₃)(CO₃)rather than production of soluble zinc ammine complexes. For example,low introduction of ammonia to the reaction medium is optionally used toprevent production of soluble zinc ammine complexes, which is preferablyachieved by a urea hydrolysis system. Upon heating above approximately90° C., the aqueous solution of urea decomposes to cyanate and ammoniumions resulting in a series of acid/base reactions. The overall effect ofthose reactions is slow release of ammonia to the reaction medium:OC(NH₂)₂

NCO⁻+NH₄ ⁺  (2)NCO⁻+2H₂O→NH₃+HCO₃ ⁻  (3)

The urea hydrolysis reactions and associated acid/base equilibria whenthe aqueous urea solution is heated at approximately 90° C. are asfollows:OC(NH₂)₂

NCO⁻+NH₄ ⁺  (4)NCO⁻+2H₂O→NH₃+HCO₃ ⁻  (5)NCO⁻+2H₂O+HCO₃ ⁻→NH₃+2HCO₃ ⁻  (6)NCO⁻+2H₂O+NH₃→2NH₃+HCO₃ ⁻  (7)HNCO+H₂O+H⁺→NH₄ ⁺+CO₂  (8)HNCO+H₂O→NH₃+CO₂  (9)H₂O

H⁺+OH⁻  (10)NH₄ ⁺

NH₃+H⁺  (11)CO₂+H₂O

HCO₃ ⁻+H⁺  (12)HCO₃ ⁻

CO₃ ⁻²+H⁺  (13)HNCO

NCO⁻+H⁺  (14)

Reactions 4 to 9 are independent reactions of decomposition of the ureato ammonia and cyanate and later decomposition of cyanate to ammonia andcarbonic acid. CO₂ in Equations 8 and 9 denotes gaseous and aqueouscarbon dioxide and carbonic acid. Equations 10 to 14 are acid/baseequilibria associated with the dissociations of urea products includingcyanic acid, carbonic acid, and ammonium ion. The overall acid/basedissociation leads to a slow release of ammonia in the solution.

The products of the second reaction (equation 3) preferably supply theammonia and carbonate to the Zn(NH₃)(CO₃) structure and the ammonium ionfrom the first reaction (equation 2) serves as buffer to maintain the pHin the range of 8 to 11. The overall reaction of crystal formationfollows:Zn²⁺+OC(NH₂)₂+3H₂O→Zn(NH₃)(C₃)+NH₄OH+2H⁺  (15)

Another function of slow release of NH₃ to reaction medium is preferablybalancing the nucleation vs. growth reactions of crystallization infavor of growth reaction that preferably leads to the formation of largesingle crystals.

INDUSTRIAL APPLICABILITY

The invention is further illustrated by the following non-limitingexample.

Example 1

A mixture of 19 mL DMF and 1 mL water was prepared. 439.0 mg (0.2 mmol)zinc acetate and 132 mg (2.2 mmol) urea were gently mixed in 2×10 mL ofthe solvent mixture to dissolve. Then, zinc acetate solution wasgradually added to the urea solution and gently mixed to make a uniformsolution. The solution of reactants was then capped and heated to 90° C.The heating resulted in precipitation. The mixture was heated for 2 hr.Next, the reaction mixture was placed into the Teflon liner of ahydrothermal reactor and the mixture was heated at 140° C. for 4 days.The process led to the formation of large single crystals of the productthat were washed with DMF and dispersed into 10 mL of fresh chloroformfor 4 days. The chloroform was exchanged every day. Finally, thecrystals were activated by heating at 110° C. under vacuum to beprepared for adsorption analysis.

Example 2

Urea (60 mg, 1 mmole) was dissolved in DMF (5 mL) in a 20-mL vial. Then,deionized water (1 mL) was slowly dripped into the urea solution in DMFand the vial was capped. The obtained solution was heated at 90-95° C.for 6 h. Afterwards, zinc acetate (329.3 mg, 1.5 mmole) was dissolved inDMF (10 mL) and dripped into the urea solution. The mixture started toprecipitate, and it was heated at 90° C. for 1 h until precipitation wascompleted. Next, the uncapped vial of the mixture was placed into theTeflon liner of a hydrothermal reactor and heated at 150° C. for 4 days.Finally, the produced crystals were washed 3 times with DMF (10 mL) andthen soaked in chloroform (10 mL) for 3 days. Every day, the usedchloroform was replaced with a fresh solvent.

FIG. 1 is a photograph of Zn(NH₃)(CO₃) single crystals.

Referring now to FIG. 2, the pattern of powder x ray diffraction(“PXRD”) analysis performed on an “as-synthesized” sample and thesimulated pattern of a phase pure product are shown. The pattern of theproduct repeated the simulated pattern. Therefore, the product was phasepure Zn(NH₃)(CO₃).

The structure of Zn(NH₃)(CO₃) is represented in FIG. 29a-c . As revealedby SC-XRD from ICSD #41113, Zn(NH₃)(CO₃) has a helical crystallineframework, which is reproduced in FIG. 29a . The structure consists ofCO₃ trigonal planars, each of which is isolated from the others by threeZnO₃N tetrahedra that share oxygens in the corners, whereas each ZnO₃Ntetrahedron keeps the N corner unshared. Regardless of polar-polarinteractions between O and H and between N and H of the adjacenttetrahedra, the tetrahedra are also isolated from each other. With thisconfiguration of isolated tetrahedra and trigonal planars, Zn, C, and Ocreate two types of folded helices that develop in the b direction(shown in orange and blue in FIG. 29a, b ): 1) a small helix of(ZnOCO)₂, and 2) a big helix of (ZnOCO)₄ with two ammines pendant fromevery other zinc toward the axis of the big helix, whose nitrogens are4.018 Å away from each other. The helices share the ZnOC piece of theirbackbones (shown in yellow in FIG. 29a, b ) and the pitches of bothhelices are 5.498 Å, which is equal to the b dimension of theZn(NH₃)(CO₃) unit cell. The spiral direction of the helices in the cdirection stays the same, but helices are packed with aright-handed/left-handed pattern in the a direction. The projection ofthe structure in the b direction visualizes two types of microchannelswith 8 and 16 components, which are shown in yellow and cyan,respectively, in FIG. 29 c.

Adsorption Isotherms

As can be seen from FIG. 29, the pendant ammines in the (ZnOCO)₄ helixhave enough distance between them to accommodate small gas moleculessuch as H₂, CO₂, O₂, N₂, and CH₄, whose kinetic diameters range from 2.9to 3.8 Å. The configuration of the (ZnOCO) in helices with a pitch of5.498 Å could make the volume of the (ZnOCO)₂ helix accessible from thea and c directions for the molecules that can primarily diffuse to the(ZnOCO)₄ helix. Based upon these speculations, an adsorption propertyfor this structure could be hypothesized. The adsorptives used foradsorption analysis in this study were CO₂, N₂, H₂, O₂, and CH₄. Thephysical properties of the adsorptives are tabulated in Table 3.1. Thekinetic diameter is a critical property of the adsorptives that iseffective in specifying the mechanism of adsorption. Polarizability isalso influential on the strength of interaction between adsorbate andadsorbent in thermodynamic mechanism. Additionally, dipole moment is asignificant measure for determining the strength of the interactionbetween adsorbate and adsorbent. Since the adsorptives of this study areall nonpolar, their dipole moments are zero.

TABLE 0.1 Physical properties of adsorptives. Molecular Boiling KineticPolarizability Adsorptive weight (amu) point (K) diameter (Å) ×10²⁵(cm³) H₂ 2.016 20.27 2.89 8.042 N₂ 28.013 77.35 3.80 17.403 O₂ 31.99990.17 3.46 15.812 CO₂ 44.01 216.55 3.3 29.11 CH₄ 16.034 111.66 3.7625.93

Referring to FIGS. 3-7, the isotherms of CO₂ uptake at 253, 263, 273,293, and 313 K and 0 to ˜4500 mmHg and N₂, H₂, CH₄, and O₂ uptake at253, 263, 273, and 293 K and 0 to ˜4500 mmHg are shown. FIG. 8 shows theadsorption branches of all of these isotherms to allow better comparisonof the adsorption behavior. The adsorption branches of the N₂, H₂, CH₄,and O₂ isotherms were convex toward the pressure axis, indicating poorinteraction between these nonpolar adsorbates and the microchannels ofthe adsorbent. The isotherm of CO₂ uptake, in contrast, was concave tothe pressure axis at lower pressures and deflects toward the uptake axisat approximately 1800 mmHg, indicating favorable adsorbate-adsorbentinteraction at lower pressures. While CO₂ had a kinetic diameter of 3.3Å, which was larger than the diameter of H₂ (2.9 Å) and smaller than N₂(3.6 Å), the hydrogen-bond-type interaction between oxygens of CO₂ andhydrogens of pendant NH₃ gave rise to the shape of the adsorption branchof the isotherm as well as the uptake, 0.56 mmol/g, which was an orderof magnitude higher than that of H₂, O₂, CH₄ and N₂.

Henry's Constants and Selectivities

Henry's constant, K_(H), which is the ratio of uptake to pressure atequilibrium and at a concentration of the adsorbate that is sufficientlylow to ignore adsorbate-adsorbate interaction (Equation 1.37),

$\begin{matrix}{q = {\frac{{K^{\prime}(T)}_{H}p}{RT} = {{K_{H}(T)}_{p} = {{K^{\prime}(T)}_{H}C}}}} & {{Eq}.\mspace{14mu} 0.1}\end{matrix}$provides a quantitative measure for assessing the interaction of anadsorbate with an adsorbent and evaluating the selectivity of thesynthesized material (Equation 1.67).

$\begin{matrix}{\alpha_{Hij} = \frac{K_{Hi}}{K_{Hj}}} & {{Eq}.\mspace{14mu} 0.2}\end{matrix}$Henry's constant can be calculated by fitting the isotherm data in avirial equation, expressed in Equation 1.51.

$\begin{matrix}{\frac{p}{q} = {\frac{1}{K_{H}} \cdot {\exp\left( {{C_{1}q} + {C_{2}q^{2}} + \ldots} \right)}}} & {{Eq}.\mspace{14mu} 0.3}\end{matrix}$For low pressures and/or small uptakes, second and higher orders of q inEquation 1.51 are small enough to be ignored. Therefore, Equation 1.51can be modified as follows:

$\begin{matrix}{{{Ln}\left( \frac{P}{q} \right)} = {{C_{1}q} - {\ln\; K_{H}}}} & {{Eq}.\mspace{14mu} 0.4}\end{matrix}$

By performing linear regression of Ln(P/q) versus q and by extrapolatingthe fitted line to calculate the intercept, K_(H) can be obtained. Thecalculated Henry's constants are shown in Table 3.4.

Henry's constant, derived from the adsorption isotherm of pure gas, isan asset for assessing the separation behavior of the material when itis exposed to a gas mixture. Table 3.5 shows the equilibrium separationselectivities of the material when it is exposed to the binary mixturesof CO₂, N₂, H₂, O₂, and CH₄ based on the ratio of Henry's constants(Equation 1.67).

Since adsorption at low temperatures is diffusion controlled theadsorbent does not offer significant equilibrium selectivity at 253 K.Rather, separation at this temperature should be based on adsorptionkinetic. At 263 K, where adsorptions on CO₂, N₂, and H₂ are controlledby both diffusion and adsorption, the equilibrium selectivities of thosebinary mixtures increase.

TABLE 0.2 Henry's constants of CO₂, N₂, H₂, O₂, and CH₄ adsorption,calculated from virial isotherm. K_(H) (mmole/kPa · g) × 10⁻⁴ 293 273263 253 Adsorptive (K) (K) (K) (K) CO₂ 5.700 5.939 5.710 28.342 N₂ 0.0910.036 0.166 15.742 H₂ 0.184 0.079 1.361 15.663 O₂ 0.506 0.634 12.71727.832 CH₄ 0.158 0.036 12.804 15.361

TABLE 0.3 Estimated selectivities for binary mixtures of CO₂, N₂, H₂,O₂, and CH₄ by Zn(NH₃)(CO₃) at 293, 273, 263, and 253 K. Gas α_(Hij)Mixture 293 (K) 273 (K) 263 (K) 253 (K) CO₂/CH₄ 36.0 166.6 0.4 1.8CO₂/H₂ 31.1 75.1 4.2 1.8 CO₂/N₂ 62.9 166.0 34.3 1.8 CO₂/O₂ 11.3 9.4 0.41.0 H₂/N₂ 2.0 2.2 8.2 1.0 O₂/N₂ 5.6 17.7 76.5 1.8

The maximum selectivity for air separation (N₂/O₂ mixture) can beachieved at 263 K, where O₂ adsorption is still in diffusion-controlleddomain, but where N₂ has entered the dual-controlled mode. Separation ofCO₂ from O₂ and CH₄ at this temperature is not advantageous because ofCO₂ adsorption being in dual-controlled mode. The largest equilibriumselectivities for CO₂ separation are exhibited at 273 K, whereadsorptions of all gases are in the dual-controlled domain.Selectivities of CO₂ separation from N₂, H₂, and CH₄ at 293 K havedecreased compared to 273 K because uptakes of those gases haveincreased at 293 K in comparison to 273 K (see the inset of FIG. 7.5).

Selectivity Based on IAST

As explained in Subsection 1.6.8.2, IAST is capable of predicting thecomposition of the adsorbed phase for a given binary mixture ofadsorptives with known q=f(p) isotherm data of pure components of themixture. Once the composition of adsorptive and adsorbate phases isknown, selectivity (α_(Iij)) can be calculated. This section calculatesα_(Iij) for selected binary mixtures with potential industrialsignificance (Table 3.6) at varied pressures (0 to 4600 mmHg) andtemperatures (293 and 273 K) to evaluate the functionality of theselectivity with pressure and temperature.

To find q=f(p), isotherm data were fitted into polynomial equations, andthe equations were plugged into Equation 1.71.

$\begin{matrix}{{\int_{t = 0}^{P\;\frac{y_{1}}{x_{1}}}{{q_{1}(t)}\frac{dt}{t}}} = {\int_{t = 0}^{P\;\frac{y_{2}}{x_{2}}}{{q_{2}(t)}\frac{dt}{t}}}} & {{Eq}.\mspace{14mu} 0.5}\end{matrix}$

The integrals of Equation 1.71 can be solved for a sample polynomialq=f(p) with n=3 as follows:

$\begin{matrix}{{\int_{t = 0}^{P\;\frac{y_{1}}{x_{1}}}{\left( {{at}^{3} + {bt}^{2} + {ct}} \right)\frac{dt}{t}}} = {{\frac{a}{3}\left( {P\;\frac{y_{1}}{x_{1}}} \right)^{3}} + {\frac{b}{2}\left( {P\;\frac{y_{1}}{x_{1}}} \right)^{2}} + {c\left( {P\;\frac{y_{1}}{x_{1}}} \right)}}} & {{Eq}.\mspace{14mu} 0.6}\end{matrix}$Equation 1.71, with the solution in the form of Equation 3.21, can besolved by trial and error for a given P to calculate equivalent x₁ (andthus x₂).

TABLE 0.4 Binary mixtures of CO₂, N₂, H₂, O₂, and CH₄ and theirpotential industrial significance considered for calculation of α_(Iij).Composition Gas Mixture (%/%) Significance CO₂/CH₄ 10/90 Natural gaspurification CO₂/CH₄ 50/50 Landfill gas separation CO₂/H₂ 40/60 Syngasseparation CO₂/N₂ 10/90 Flue gas separation CO₂/O₂ 90/10 Oxy-fuel gasseparation H₂/N₂ 50/50 —

FIG. 9 shows the varied compositions of the adsorbed phase and theseparation selectivities by Zn(NH₃)(CO₃) with pressure at 273 K and 293K for a binary mixture of CO₂/CH₄ (10/90), FIG. 10 shows the same forCO₂/CH₄ (50/50), FIG. 11 for CO₂/H₂ (40/60), FIG. 12 for CO₂/N₂ (10/90),FIG. 13 for CO₂/O₂ (90/10), and FIG. 14 for H₂/N₂ (50/50).

The common feature of all separations is an exponential decrease ofselectivity and therefore, the composition of more adsorbed adsorptiveas pressure—i.e., uptake—increases. This behavior can be attributed tothe effect of adsorbate-adsorbate interaction in the selectivity ofadsorbent as uptake increases.

Selectivity, as a ratio of Henry's constants of the separation mixture,(α_(Hij)), is a measure of the adsorbent's behavior at low pressures.The values of α_(Hij) for separation by Zn(NH₃)(CO₃) were calculated andtabulated in Table 3.5. To compare the selectivity based on Henry'sconstant and IAST, α_(Iij) at 10 mmHg and α_(Hij) at 293 K and 273 K arecompared in Table 3.7.

The values of α_(Iij) and α_(Hij) at 293 K and 273 K for separation ofCO₂/CH₄ (10/90) are far different. On the other hand, α_(Iij) andα_(Hij) have the same order of magnitude at 293 K and 273 K for theseparation of the same components but different concentrations: CO₂/CH₄(50/50). This discrepancy of selectivities for the separation of 10/90mixture is related to the inaccuracy of IAST method for predicting thecomposition of the adsorbed phase of the mixture with highconcentrations of the less-adsorbed component (here, CH₄ withconcentration of 90%). A similar discrepancy can be observed betweenα_(Iij) and α_(Hij) for the separation of CO₂/N₂ (10/90). The agreementbetween α_(Iij) and α_(Hij) increased as the concentration of theless-adsorbed component decreased, and the highest agreement wasobserved for CO₂/O₂ (90/10), where at 293 K, α_(Iij) and α_(Hij) are11.3 and 13.8, respectively, and 9.4 and 9.2, respectively, at 273 K.

At low pressures, IAST predicts that the concentration of adsorbed CO₂and consequently the selectivity for the separation of CO₂/H₂ (40/60)and CO₂/N₂ (10/90) at 273 K are smaller than at 293 K (see FIGS. 11 and12). However, the trends reverse as pressure increases (approximately700 mmHg for CO₂/H₂ (40/60) and 150 mmHg for CO₂/N₂ (10/90)). Suchchanges in selectivity and the concentration of the adsorbed phase arecongruent with H₂ and N₂ isotherms at 293 K and 273 K (see FIGS. 4 and3). The values of α_(Hij) at 293 K and 273 K agree with the IAST trendat higher pressure (α_(Hij) at 273 is larger than α_(Hij) at 293 K).That is because the uptakes at larger pressures were used to calculateα_(Hij).

As FIG. 13 shows, IAST predicts the concentration of adsorbed CO₂ andthe selectivity of CO₂/O₂ (90/10) separation at 293 K to be larger thanthose at 273 K. This trend agrees with the trend of α_(Hij) at 293 K and273 K represented in Table 3.5.

TABLE 0.5 Comparison of α_(Hij) and α_(Iij) at 293 K and 273 K fordifferent binary mixtures. Gas Mixture 293 (K) 273 (K) (%/%) α_(Hij)α_(Iij) ^(a) α_(Hij) α_(Iij) ^(a) CO₂/CH₄ (10/90) 36.0 1491.0 166.61583.9 CO₂/CH₄ (50/50) 36.0 44.5 166.6 351.1 CO₂/H₂ (40/60) 31.1 22.775.1 19.1 CO₂/N₂ (10/90) 62.9 520.4 166.0 152.1 CO₂/O₂ (90/10) 11.3 13.89.4 9.2 H₂/N₂ (50/50) 2.0 6.3 2.2 9.4 ^(a) Calculated at 10 mmHgBET Analysis and Structural Stability

To calculate the BET surface area, N₂ adsorption analysis byZn(NH₃)(CO₃) was performed at 77 K. FIG. 28 shows the adsorptionisotherm of the material. 5.3 mmole/g of N₂ was adsorbed at the relativepressure (P/P_(s)) of 0.995. The adsorption branch of the isotherm isalmost linear, and hysteresis at lower pressure could still be observed.

The equivalent BET surface area calculated for the Zn(NH₃)(CO₃)framework is 207 m²/g. The C-value and correlation coefficient werecalculated as 2.44 and 0.9973, respectively. The C-value, a parameterused for fitting the BET equation, is exponentially related to theenthalpy of adsorption in the first layer of adsorbed gas, and it is aqualitative measure for adsorbate-adsorbent interaction. The obtainedBET surface area is in the range of the values reported for otherultramicroporous materials.

To assess if the material is sufficiently structurally robust towithstand the recurring cycles of adsorption and desorption thatcommonly occur in separation processes, a comparison was performedbetween the CO₂ adsorption isotherms of a sample of fresh Zn(NH₃)(CO₃)and Zn(NH₃)(CO₃) subjected to 80 cycles of adsorption and desorption.The maximum uptake at 4500 mmHg for the used sample was only 0.8% lessthan the uptake of the fresh sample, indicating that the materialstructure is stable under the cyclical adsorption-desorption process.

In addition to structural stability, as the adsorption analysisprocedure implies, Zn(NH₃)(CO₃) is thermally stable at 110° C. To checkthe chemical stability in water, the crystals were mixed in water forweeks, a procedure that was found not to affect Zn(NH₃)(CO₃) structure.

Potential Industrial Applications

As previously discussed there are CO₂ separation processes associatedwith power generation, in which CO₂/N₂, CO₂/H₂, and CO₂/O₂ are goodconstituents of post-combustion, pre-combustion, and oxy-fuel CO₂capture, respectively. In addition to power generation sector, thenatural gas industry is another sector facing the challenge of CO₂separation. An important component of natural gas is CH₄ (80-95%), andit also includes impurities composed of N₂, CO₂, H₂, other hydrocarbonsheavier than CH₄, and traces of other materials such as water andsulphur. Landfill gas (LFG) is another source of CH₄. LFG comprises45-60% CH₄ and 40-60% CO₂. CO₂ separation from CH₄ is necessary toupgrade the natural gas and prevent pipeline corrosion.

As explained above, Zn(NH₃)(CO₃) is thermally, chemically, andstructurally robust enough to withstand the operating conditions of CO₂separation processes. In the present subsection, the adsorptionselection criteria are calculated for Zn(NH₃)(CO₃) for theabove-mentioned CO₂ separation processes by Pressure swing adsorption(“PSA”) or Vacuum swing adsorption (“VSA”), and the values are comparedwith those of other adsorbents. For each process, a binary mixture withan average composition is selected, and adsorption pressure (P_(ads))and desorption pressure (P_(des)) are set close to the conditions ofupstream industrial operations. The criteria used to evaluate theadsorbents' performances are uptake capacity (q), working capacity (Δq),working selectivity (α_(qij)), regenerability (R), and adsorbentselection parameter (S). Out of this analysis and comparison, thesuitability of the adsorbent for a specific application can be assessed.

Natural Gas Purification Using PSA

Typical operating conditions for natural gas purification using PSA areas follows:

-   -   CO₂/CH₄ composition: 10/90    -   P_(ads)/P_(des): 5 atm/1 atm    -   T: Room temperature (RT)

Table 3.9 tabulates the calculated values of the selection parametersfor Zn(NH₃)(CO₃) and other prominent adsorbents. Zn(NH₃)(CO₃) exhibitsthe highest R, which is the result of the type of isotherm. However,since the partial pressure of the CO₂ in the mixture is low, q and Δq ofthe adsorbent are lower than other adsorbents. Moreover, otheradsorbents are better candidates with respect to the working selectivityand S. All in all, Zn(NH₃)(CO₃) is not the best candidate for PSA-basednatural gas purification.

TABLE 0.6 Performance of Zn(NH₃)(CO₃) and other selected adsorbents inPSA-based natural gas purification. The highest value of each parameteris bolded. T q Δq R Adsorbent (K) (mmole/g) (mmole/g) (%) α_(qij) SZn(NH₃)(CO₃) 293 0.027 0.022 80.9 7.2 1.4 POP1^(a) 298 1.39 0.86 62.29.7 7.5 MOF1^(b) 303 0.89 0.62 69.7 16.7 18.7 HKUST-1 298 2.07 1.70 63.010.0 9.6 MOF Zeolite-13X 298 3.97 1.48 37.3 18.9 9.0 ^(a)Diimide porousorganic polymer (POP) ^(b)Amine-Al-MIL-53LFG Separation Using PSA

Typical operating conditions for this process are as follows:

CO₂/CH₄ composition: 50/50

-   -   P_(ads)/P_(des): 5 atm/1 atm    -   T: RT

Table 3.10 tabulates the calculated values of the selection parametersfor Zn(NH₃)(CO₃) and other prominent adsorbents. Regardless of q and Δq,Zn(NH₃)(CO₃) provides higher R, α_(qij), and S values than other knowncandidates. Therefore, Zn(NH₃)(CO₃) can be an appropriate adsorbent forLFG separation by PSA.

TABLE 0.7 Performance of Zn(NH₃)(CO₃) and other selected adsorbents inPSA-based LFG separation. The highest value of each parameter is bolded.T q Δq R Adsorbent (K) (mmole/g) (mmole/g) (%) α_(qij) S Zn(NH₃)(CO₃)293 0.158 0.131 82.9 13.5 50.7 HKUST-1 298 8.01 5.34 66.7 4.9 21.0 MOFMIL-101c 303 6.70 3.20 47.8 5.0 9.5 MOF MOF1^(a) 298 0.94 0.66 70.6 3.38.3 Zeolite-13X 298 5.37 1.40 26.1 4.2 2.0 POP1^(b) 298 2.93 1.44 49.23.6 11.5 ^(a)[Zn₃(OH)(p-cdc)_(2.5)(DMF)₄] where p-CDC is deprotonatedform of 1,12-dihydroxydicarbonyl-1,12-dicarba-closo-dode-caborane^(b)35% Li-reduced diimide-POPLFG Separation Using VSA

Typical operating conditions for this process are as follows:

-   -   CO₂/CH₄ composition: 50/50    -   P_(ads)/P_(des): 1 atm/0.1 atm    -   T: RT

Table 3.11 displays the calculated values of the selection parametersfor Zn(NH₃)(CO₃) and other prominent adsorbents. Regardless of q and Δq,Zn(NH₃)(CO₃) provides higher R, α_(qij), and S values than other knowncandidates. The superior performance of Zn(NH₃)(CO₃) for this processoriginates from the fact that CH₄ adsorption at the conditions of thisprocess can practically be considered as zero. Therefore, Zn(NH₃)(CO₃)can be an appropriate adsorbent for LFG separation by VSA.

TABLE 0.8 Performance of Zn(NH₃)(CO₃) and other selected adsorbents inVSA-based LFG separation. The highest value of each parameter is bolded.T q Δq R Adsorbent (K) (mmole/g) (mmole/g) (%) α_(qij) S Zn(NH₃)(CO₃)293 0.027 0.024 90.5 42.8 272.7 HKUST-1 298 2.81 1.90 67.5 5.5 19.8 MOFMIL-101c 303 6.70 3.20 47.8 5.0 9.5 MOF Mg-MOF-74 298 7.23 2.32 32.112.5 23.5 ZIF-82 298 1.42 1.20 84.9 5.6 20.5 Zeolite-13X 298 3.97 1.9749.6 13.2 19.1Post-Combustion Flue Gas Separation Using VSA

Typical operating conditions for this process are as follows:

-   -   CO₂/N₂ composition: 10/90    -   P_(ads)/P_(des): 1 atm/0.1 atm    -   T: RT

Regarding the low partial pressure of CO₂, the CO₂ uptake is notconsiderable under the conditions of this process. In addition,adsorbents like zeolite 13-x, Co-carborane MOF-4b, and ZIF-78 showhigher R, α_(qij), and S values than Zn(NH₃)(CO₃). Therefore,Zn(NH₃)(CO₃) is not a good candidate for VSA-based post-combustion fluegas separation.

Pre-Combustion Hydrogen Separation Using PSA

Typical operational conditions for this process are as follows:

-   -   CO₂/H₂ composition: 40/60    -   P_(ads)/P_(des): 5 atm/1 atm    -   T: RT

Table 3.12 shows the calculated values of the selection parameters forZn(NH₃)(CO₃) and other prominent adsorbents. Zn(NH₃)(CO₃) exhibits thehighest R, which is result of the type of isotherm. However, since thepartial pressure of the CO₂ in the mixture is low, q and Δq of theadsorbent are lower than other adsorbents. Moreover, other adsorbentsare better candidates with respect to the working selectivity and S. Allin all, Zn(NH₃)(CO₃) is not a good candidate for PSA-based natural gaspurification.

TABLE 0.9 Performance of Zn(NH₃)(CO₃) and other selected adsorbents inPSA-based precombustion hydrogen separation. The highest value of eachparameter is bolded T q Δq R Adsorbent (K) (mmole/g) (mmole/g) (%)α_(qij) S Zn(NH₃)(CO₃) 293 0.122 0.101 82.5 7.3 10.9 Carbon active 3033.33 1.88 56.5 58.7 639.6 Zeolite NaX 303 5.02 1.22 24.4 4.04 2.6Zeolite 303 3.93 1.96 49.8 98.3 1960.3Oxy-Fuel CO₂ Purification Using PSA

Typical operating conditions for this process are as follows:

-   -   CO₂/O₂ composition: 90/10    -   P_(ads)/P_(des): 5 atm/1 atm    -   T: RT

Table 3.13 tabulates the calculated values of the selection parametersfor Zn(NH₃)(CO₃) and other prominent adsorbents. Regardless of q and Δq,Zn(NH₃)(CO₃) provides higher R, α_(qij), and S values than other knowncandidates. The superior performance of the adsorbent for this processoriginates from the high partial pressure of CO₂ and low O₂ uptake atRT. Therefore, Zn(NH₃)(CO₃) can be an appropriate adsorbent for Oxy-fuelCO₂ Purification by PSA.

TABLE 0.10 Performance of Zn(NH₃)(CO₃) and other selected adsorbents inPSA-based oxy-fuel CO₂ purification. The highest value of each parameteris bolded. T q Δq R Adsorbent (K) (mmole/g) (mmole/g) (%) α_(qij) SZn(NH₃)(CO₃) 293 0.338 0.288 85.1 17.3 844.8 Cu-BTC MOF 298 10.207 5.76056.4 4.1 6.6 Zeolite NaX 303 5.548 1.281 23.1 9.4 85.5

Regarding the ultramicroporous structure, Zn(NH₃)(CO₃) does not exhibitlarge gas uptake compared to other adsorbents. However, this deficiencyis offset by considerable selectivity, regenerability, and adsorbentselection parameter—especially for processes in which partial pressureof CO₂ and/or adsorption pressure is large. The origin of this behavioris the Type II isotherm of CO₂ adsorption and the Type II isotherm ofadsorption of other gases at room temperature. Since all selectioncriteria are considered for industrial adsorbent selection, Zn(NH₃)(CO₃)is capable of being adopted for processes such as LFG and CO₂/O₂separations. It should be noted that chemical, structural, and thermalstabilities were not assessed for all the candidate adsorbentsreferenced in Tables 3.9 to 3.13. If these conditions were considered inthe selection criteria, some adsorbents would be recognized asunsuitable for industrial use. For instance, MOF-74's superior CO₂capacity is due to the exposed cations, which make MOF-74 water- andair-sensitive and thus inapplicable for LFG (see Table 3.11) and fluegas separation.

N₂ and O₂ needed in industries are acquired from air by air separationprocesses, and the most common air separation process utilized inindustry is cryogenic distillation. Since the critical temperature andpressure of air are −140.7° C. (132.5 K) and 37.7 bar, respectively, airseparation should be performed at temperatures below −140.7° C. (132.5K). For instance, to separate air at atmospheric pressure, thetemperature must be decreased to −192° C. (81.5 K) to have vapor-liquidequilibrium. To increase this temperature to −172° C. (101 K), airshould be compressed to 6 bar. Therefore, air separation by this processis extremely energy-intensive. On the other hand, air separation by theadsorption process is limited to small-scale separation by zeolitesunder the kinetic mechanism. The O₂ and N₂ separated by this process arenot pure and usually are used in hospitals. As Table 3.5 implies,Zn(NH₃)(CO₃) shows considerable selectivity to separate O₂ from N₂ at263 K (α_(Hij)=76.5). Although this temperature is stillsub-atmospheric, it is much higher than temperatures at which cryogenicdistillation is operated. Therefore, adsorption-based air separation byZn(NH₃)(CO₃) can be proposed, for which the operating conditions are asfollows:

-   -   O₂/N₂ composition: 20/80    -   P_(ads)/P_(des): 5 atm/1 atm    -   T: 263 K

Table 1.4 tabulates the calculated values of the selection parametersfor Zn(NH₃)(CO₃). Based upon these selection criteria, it can beexpected that air separation by this process is less energy-intensivethan cryogenic distillation. To validate the proposed process, energyassessment analysis is required to measure the energy penalty associatedwith this process and compare it with that of cryogenic distillation.Such analysis was out of scope of this study.

TABLE 0.11 Performance of Zn(NH₃)(CO₃) in a PSA based air separation. qΔq R (mmole/g) (mmole/g) (%) α_(qij) S 0.150 0.121 80.8 19.2 49.5

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/orparameters of this invention for those used in the preceding examples.

Note that in the specification and claims, “about” or “approximately”means within twenty percent (20%) of the numerical amount cited.Although the invention has been described in detail with particularreference to these embodiments, other embodiments can achieve the sameresults. Variations and modifications of the present invention will beobvious to those skilled in the art and it is intended to cover in theappended claims all such modifications and equivalents. The entiredisclosures of all references, applications, patents, and publicationscited above are hereby incorporated by reference.

What is claimed is:
 1. A method for capturing CO₂ in a fixed bed columncomprising: adsorbing CO₂ from a gas mixture stream in a columncomprising Zn(NH₃)(CO₃) sorbent at room temperature and at an adsorptionpressure P_(ads); and regenerating the column adsorbing the CO₂, afterits saturation breakthrough, by desorption at room temperature and at adesorption pressure P_(des), wherein P_(des) is less than P_(ads). 2.The method of claim 1 wherein the gas mixture is a CO₂/O₂ mixture. 3.The method of claim 2 wherein the capture is for an oxy-fuel CO₂ processby pressure swing adsorption.
 4. The method of claim 3 wherein theP_(ads)/P_(des) is about 5 atm/1 atm.
 5. The method of claim 1 whereinthe gas mixture is CO₂/CH₄.
 6. The method of claim 5 wherein the captureis for a landfill gas process under pressure swing adsorption.
 7. Themethod of claim 5 wherein the P_(ads)/P_(des) is about 5 atm/1 atm. 8.The method of claim 5 wherein the capture is for a landfill gas processunder pressure swing adsorption.
 9. The method of claim 8 wherein theP_(ads)/P_(des) is about 1 atm/vacuum pressure of 0.1 atm.